Mobile communication utilizes the features of wireless communication to the maximum extent. It has been widely used in public communication such as ship telephone and train telephone as well as police telephone, press telephone and taxi telephone. As the social activities diverse, needs for the mobile communication services increase. Among others, the need is high for a public mobile radio telephone system which enables telephone communication between a mobile passenger and an ordinary telephone subscriber or between mobile passengers. The public mobile radio telephone service started at St. Louis in the U.S.A. in 1949 with a small scale of ten channels using 150 MHz band. Thereafter, in 1961, investigation was made for a 400 MHz band mobile radio telephone system and the result was put into practice in 1971 at San Fernando near Los Angeles when they had an earthquake there. In 1976, as technologies were developed such as effective utilization of frequency by small zone configuration, 600 channel switching anti-vibration synthesizers, highly stable crystal oscillators, busy channel switching (tracking switching) in combination with an electronic telephone exchange and location registration, the mobile radio telephone system has come into commercial usage.
When a driver of a mobile having a mobile radio telephone system or so-called wireless telephone system installed therein receives a call from a fixed station telephone subscriber while he is driving the car, he has to operate the telephone system too. The operation of the telephone system should disturb the driving operation of the driver as little as possible. For this purpose, a telephone system with a hang-type handset in which the handset is placed near the face of the driver so that the driver need not handle the handset by hand may be advisable. In this case, however, some means is required to prevent the problem of a voice going from a speaker serving as a telephone receiver to a microphone serving as a telephone transmitter during the telephone reception and transmission. It is difficult, however, to sufficiently prevent this problem in the closed mobile space because of echo.